Genetic diversity and gene exchange in Pinus oocarpa, a Mesoamerican pine with resistance to the pitch canker fungus (Fusarium circinatum)
Eleven highly polymorphic microsatellite markers were used to determine the genetic structure and levels of diversity in 51 natural populations of Pinus oocarpa across its geographic range of 3000 km in Mesoamerica. The study also included 17 populations of Pinus patula and Pinus tecunumanii chosen for their resistance or susceptibility to the pitch canker fungus based on previous research. Seedlings from all 68 populations were screened for pitch canker resistance, and results were correlated to mean genetic diversity and collection site variables. Results indicate that P. oocarpa exhibits average to above-average levels of genetic diversity (A ¼ 19:82, AR ¼ 11:86, HE ¼ 0:711) relative to other conifers. Most populations were out of Hardy-Weinberg equilibrium, and a high degree of inbreeding was found in the species (FIS ¼ 0:150). Bayesian analysis grouped P. oocarpa into four genetic clusters highly correlated to geography and distinct from P. patula and P. tecunumanii. Historic gene flow across P. oocarpa clusters was observed (Nm ¼ 1:1–2:7), but the most pronounced values were found between P. oocarpa and P. tecunumanii (low-altitude provenances) in Central America (Nm ¼ 9:7). Pinus oocarpa appears to have two main centers of diversity, one in the Eje Transversal Volca´nico in central Mexico and the other in Central America. Introgression between P. oocarpa and P. tecunumanii populations appears to be common. Pinus oocarpa populations showed high resistance to pitch canker (stemkill 3%–8%), a disease that the species has presumably coevolved with in Mesoamerica. Resistance was significantly correlated to the latitude, longitude, and altitude of the collection site but not to any genetic-diversity parameters or degree of admixture with P. tecunumanii.